Molds have conventionally been produced by mixing a refractory particulate material with a binder composition, packing the resulting mixed sand into a pattern, and then curing a binder in the binder composition. As the binder, various curable resins such as furan resins, water-soluble phenolic resins, and urethane resins have been used. Of these, water-soluble phenolic resins cure with an organic ester or carbon dioxide gas, thus they are recognized as a binder which can prevent the working atmosphere during the production of self-curing or gas-curing molds from worsening (JP-A-50-130627, JP-B-61-43132, and JP-B-61-37022). (The terms "JP-A" and "JP-B" as used herein mean an "unexamined published Japanese patent application" and an "examined Japanese patent publication", respectively.) The water-soluble phenolic resins herein means alkaline phenol-formaldehyde resins obtained by subjecting phenol and formaldehyde to polycondensation in the presence of an alkali or by adding an alkali after the polycondensation of these two compounds.
However, the production of molds from a mixed sand obtained by mixing a refractory particulate material with a binder composition containing such an alkaline phenol-formaldehyde resin has been defective in that, as compared with the case of using a binder composition containing a furan resin, the mixed sand has such poor flowability that it cannot be densely packed into a pattern. A mixed sand with poor flowability has a drawback that uniform mixing of the refractory particulate material with the binder is difficult. Further, use of a mixed sand with poor pattern-filling properties has often caused a rough surface or a reduced surface strength of the resulting mold. Accordingly, there has been a drawback that when a mold obtained using an alkaline phenol-formaldehyde resin as a binder is used in the production of a mold, the resulting cast has defects such as sand inclusion, burning, penetration, and buckle, thus causing a decrease in cast quality.
Hence, it has been proposed to incorporate, as a flow accelerator, a fluorine-compound surfactant (JP-A-2-299741) or an aliphatic alcohol (JP-A-3-134067) into a binder composition containing an alkaline phenol-formaldehyde resin. However, fluorine-compound surfactants and aliphatic alcohols have poor compatibility with alkaline phenol-formaldehyde resins and have failed to sufficiently improve the flowability of the mixed sands. In addition, fluorine-compound surfactants and aliphatic alcohols have had another drawback that they are relatively expensive, which makes the resulting casts expensive.
On the other hand, it has been proposed to use, as a binder, a phenolic resin obtained by polycondensation of formaldehyde with a bisphenol in place of phenol (JP-A-62-40948 and JP-A-63-40636). In this technique, bisphenol A, bisphenol F, bisphenol C, or the like is used alone as the bisphenol and, hence, the resulting phenolic resin is a binary polymer formed by polycondensation of the bisphenol and formaldehyde.
The strength of molds obtained using this bisphenol-formaldehyde binary polycondensation resin improves at a certain degree, but it is not yet been fully satisfactory. That is, there has been a drawback that, although a fully satisfactory mold strength is obtained when the temperature of the refractory particulate material to be mixed with the resin (sand temperature) is high, a low sand temperature results in a considerable decrease in the mold strength. The binder resin has another drawback that a considerable decrease in the mold strength is caused when the amount of a hardener is reduced.
It has also been proposed to use, as a binder, a phenolic resin obtained by polycondensation of formaldehyde with a polyhydric phenol used in place of phenol (JP-A-1-166853). However, even with the proposed technique, a sufficient improvement in the strength of molds obtained has not yet been attained.